1. Field of the Invention
This present invention relates to a method of processing seismic data, in particular to a method of processing multi-component marine seismic data in order to estimate properties of the seafloor and sensor calibration filters. It also relates to an apparatus for processing seismic data.
2. Description of the Related Art
FIG. 1 is a schematic illustration of one marine seismic surveying arrangement. In this arrangement, a seismic source 1 is towed through a water layer (in this case the sea) by a survey vessel 2, and is caused to emit discrete pulses of seismic energy. The surveying arrangement includes a seismic sensor 3, generally known as a xe2x80x9creceiverxe2x80x9d, for detecting seismic energy emitted by the source 1. In FIG. 1 the receiver 3 is disposed on the sea-bed. (A practical seismic surveying arrangement will generally include an array of more than one receiver; for example, in an Ocean Bottom Cable survey a plurality of receivers are attached to a support cable and the cable is then deployed on the sea-bed. However, the principles of a marine seismic surveying arrangement will be explained with reference to only one receiver, for ease of explanation.)
Seismic energy may travel from the source 1 to the receiver 3 along many paths. For example, seismic energy may travel direct from the source 1 to the receiver 3, and this path is shown as path 4 in FIG. 1. Path 4 is known as the xe2x80x9cdirect pathxe2x80x9d, and seismic energy that travels along the direct path 4 is known as the xe2x80x9cdirect wavexe2x80x9d.
Another path of seismic energy from the source 1 to the receiver 3 involves a single reflection at a reflector 7 disposed within the earth, and this is shown as path 5 in FIG. 1. (This path will also involve refraction at the sea-floor and at interfaces between different layers within the earth, but this has been omitted for clarity.) This path is known as the xe2x80x9cprimary pathxe2x80x9d, and seismic energy received at the receiver 3 along this path is known as the xe2x80x9cprimary reflectionxe2x80x9d. Only one reflector is shown in FIG. 1, but typical seismic data will contain primary reflections from many different reflectors within the earth.
Not all downwardly-propagating seismic energy that is incident on the sea-bed will pass into the earth""s interior, and a proportion will be reflected upwards back into the sea. Furthermore, the source 1 may emit some upwardly-propagating seismic energy which will reach the receiver after undergoing reflection at the sea-surface. These effects give rise to seismic energy paths, for example such as paths 6a and 6b in FIG. 1, that involve more than pass through the water. These paths are known as xe2x80x9cwater layer multiplexe2x80x9d paths.
The existence of many paths from the source 1 to the receiver 3 in a seismic surveying arrangement of the general type shown in FIG. 1 complicates analysis of seismic data acquired by the receiver 3. When seismic data acquired by the receiver 3 are analysed, it is necessary to distinguish events arising from a primary reflection, events arising from the direct wave and events arising from a water-layer multiple. In deep water there is generally a significant time delay between an event arising from the direct wave and an event arising from a water-layer multiple, but in shallow water an event arising from a water-layer multiple may occur very shortly after an event arising from the direct wave.
A further factor that complicates the analysis of seismic data acquired by the receiver 3 is that the properties of the earth are not uniform. In particular, there is frequently a layer 8 at or near the surface whose properties may well be significantly different from the properties of the underlying geological structure 9 (hereinafter referred to as the xe2x80x9cbasementxe2x80x9d). This can occur if, for example, there is a layer at or near the earth""s surface that is less consolidated than the basement. In particular, the velocity of seismic energy may be significantly lower in the surface or near-surface layer 8 than in the basement 9, and such a surface or near-surface layer is thus generally known as a xe2x80x9clow-velocity layerxe2x80x9d (or LVL). This difference in velocity will produce a shift in the travel time of seismic energy compared to the travel time that would be recorded if the surface or near-surface layer and the basement had identical seismic properties, and these shifts in travel time are generally known as xe2x80x9cstatic shiftsxe2x80x9d, or just xe2x80x9cstaticsxe2x80x9d.
The static shift generated by a surface or near-surface low-velocity layer 8 depends on the thickness of the layer, and on the velocity of propagation of seismic energy through the layer. Lateral variations usually occur in both the thickness of a low-velocity layer 5 and the propagation velocity through the layer, so that the static shift observed at a seismic receiver at one location is likely to be different from the static shift observed at a receiver at another location. To a first approximation, the entire data set recorded at one receiver will be advanced or delayed by a static time shift relative to data recorded at another receiver.
The receiver 3 may measure only a single component of the received seismic energy. Recently, however, it has become common for the receiver 3 to record more than one component of the received seismic energy; for example, the receiver may record the x-, y- and z-components of the particle velocity and the pressure (which is a scalar quantity). interest in acquisition of multi-component seabed seismic data has increased significantly. Since multi-component seabed recordings record shear waves (S-waves), as well as P-waves, it is possible to image through sequences that are opaque to P-waves (e.g. gas-clouds). Moreover, since shear waves reveal the presence of anisotropy more clearly than P-waves, multi-component recordings can yield additional information about the physical properties of the subsurface or about the presence and orientation of small-scale fractures for instance.
Multi-component seismic data can be processed to give information about various seismic parameters, or can be separated into an up-going wavefield and a down-going wavefield. One problem encountered in processing multi-component seismic data is that incorrect sensor calibration can lead to one component of the recorded data being recorded less accurately than the other components. For example, where the receivers are mounted on a support cable, the component of particle velocity in the in-line direction (parallel to the cable) may be recorded more accurately than the component of particle velocity in the cross-line direction (perpendicular to the cable). This problem is known as xe2x80x9cvector infidelityxe2x80x9d.
There have been a number of proposals for filters that allow decomposition of multi-component seabed seismic data, for example by Amundsen, L. and Reitan, A., in xe2x80x9cDecomposition of multi-component sea-floor data into up-going and down-going P and S-wavesxe2x80x9d, Geophysics, Vol. 60, No. 2, 563-572 (1995), by Wapenaar, C.P.A et al in xe2x80x9cDecomposition of multi-component seismic data into primary P- and S-wave responsesxe2x80x9d, Geophys. Prosp., Vol. 38, 633-661 (1990), and by Amundsen, L et al, in xe2x80x9cMultiple attenuation and P/S splitting of multi-component OBC data at a heterogeneous sea floorxe2x80x9d, Wave Motion, vol 32, 67-78 (2000) and in xe2x80x9cDecomposition of multi-component sea-floor data into up-going and down-going P- and S-wavesxe2x80x9d, Geophysics, Vol. 60, No. 2, 563-572 (2000). However, these filters rely on the assumption that the data input to these schemes are a good vector representation of the actual seismic signal acquired at the receiver, and they also require knowledge of the elastic properties of the seafloor, For this reason, the issues of wavefield decomposition, statics estimation and vector fidelity are intrinsically coupled.
Knowledge of the properties of the surface layer 8 is required in a number of processing steps for multi-component seabed seismic data. These include wavefield separation, statics estimation and removal, noise attenuation and removal of water layer reverberations. Amundsen, L. and Reitan, A. have proposed, in xe2x80x9cEstimation of seafloor wave velocities and density from pressure and particle velocity by AVO analysisxe2x80x9d, Geophysics, Vol. 60, No. 5, 1575-1578 (1995), estimating the P- and S-wave velocities as well as the density of the surface layer 8 through AVO analysis of the sea-floor reflection coefficient. The P-wave velocity of the surface layer 8 can further be obtained from, for example, analysis of refracted waves. Their technique, however, does not address incorrect sensor calibration. Moreover, the accuracy of the S-wave velocity estimate is limited and can only be obtained if far-offset data is included in the inversion. Alternatively, the P-wave velocity can be obtained from analysis of refracted waves.
A method of estimating the surface layer shear velocity by means of inversion of phase-velocity of Scholte waves has been developed by Muyzert, E., in xe2x80x9cScholte wave inversion for a near-surface S-velocity model and PS-staticsxe2x80x9d, 70th Ann. Internat. Mtg. Soc. Expl. Geophys., 1197-1200 (2000). Satisfactory results have been reported for this method, for both synthetic data and field data. Its main application is for estimating PS-statics, although the S-wave velocity estimate could in principle be used for wavefield decomposition as well. However, the technique typically operates at temporal frequencies that are much lower than the typical bandwidth used in a seismic reflection survey. Therefore, it is not clear whether this technique yields estimates of the surface layer properties that can be used directly as input for the decomposition schemes.
One possible strategy to tackle the above problem has been put forward by Schalkwijk et al, in xe2x80x9cApplication of two-step decomposition to multi-component ocean-bottom data, theory and case studyxe2x80x9d, Journ. of Seism. Expl. 8, 261-278 (1999). The main principle of this method, generally known as the xe2x80x9cDelft two-step decomposition schemexe2x80x9d, is to divide the decomposition procedure into a number of smaller steps. The required information (elastic properties of the surface layer and sensor calibration) can then be obtained by imposing geophysical constraints on the intermediate decomposed results. Although this approach allows for an elastic decomposition without any a priori information about the subsurface, it requires considerable interpretation of the recorded data in advancexe2x80x94in particular it requires the prior identification of different events in the seismic data. Moreover, the identification of the desired arrivals can be particularly troublesome for some data, for example data recorded in shallow seas. This approach is, therefore, rather time-consuming and unsuitable for real-time processing applications.
U.S. Pat. No. 5,621,700 proposes directly comparing recordings of pressure and vertical component of particle velocity at each sensor package. Up-going waves could then be identified by identical polarities, whereas opposite polarities would characterise down-going energy (direct wave and water layer reverberations). Although the method may be unsuitable for many applications, good images have been obtained in some cases.
The present invention seeks to provide a method for estimating the elastic properties of the surface layer and the sensor calibration operators without the requirement of a prior step of data interpretation.
The present invention provides a method of processing multi-component seismic data acquired at a receiver, the method comprising the steps of:
a) decomposing a first portion of the seismic data into a plurality of wavefield components using a first decomposition scheme;
b) selecting first and second wavefield components that should not have arrived simultaneously at the receiver; and
c) multiplying the first wavefield component by the second wavefield component.
Since the first and second wavefield components should not arrive simultaneously at the receiver, the result of step (c) should be zero if the decomposition scheme is accurate. Step (c) thus serves to check the accuracy of the decomposition scheme used, Alternatively, the invention can be used to determine the most accurate decomposition scheme, by finding the decomposition scheme that minimises the absolute value of the result of step (c).
In a preferred embodiment the method further comprises the step (d) of adjusting one or more parameters of the decomposition scheme on the basis of the result of step (c). If the result of step (c) shows that the initial decomposition scheme was inaccurate, it is possible to adjust one or more parameters of the decomposition scheme that relate, for example, to the properties of the surface layer or to the calibration of the components of the receiver. The adjusted decomposition scheme can then be checked for accuracy and, if necessary, further adjustments can be made until a satisfactory decomposition scheme is reached (i.e., until a decomposition scheme that minimises the absolute value of the result of step (c) is found).
In an alternative preferred embodiment the method further comprises the steps of:
e) decomposing the first portion of the seismic data into a plurality of wavefield components using a second decomposition scheme;
f) multiplying the first selected wavefield component obtained in step (e) by the second selected wavefield component obtained in step (e); and
g) comparing the result of step (c) with the result of step (h).
The first embodiment provides an iterative method for adjusting the parameters of an initial decomposition scheme until it gives satisfactory results. In the second embodiment, in contrast, the seismic data are decomposed using two (or more) different decomposition schemes and the most accurate of the schemes is selected.
A second aspect of the invention provides an apparatus for processing multi-component seismic data acquired at a receiver, the apparatus comprising:
a) means for decomposing a first portion of the seismic data into a plurality of wavefield components using a first decomposition scheme;
b) means for selecting first and second wavefield components that should not have arrived simultaneously at the receiver; and
c) means for multiplying the first wavefield component by the second wavefield component